Ferroelectric liquid crystal (PLC) exhibits ferroelectricity through spontaneous polarization. It is known that liquid crystal having a permanent dipole moment in a direction vertical to the direction of the molecular long axis has a layered structure in a smectic phase, that the permanent dipole moment is not cancelled in a shift to a chiral smectic C (hereinafter referred to as SmC*) phase, in which the molecular long axis in this layered structure is tilted, even though it is averaged as a whole, and that spontaneous polarization is therefore generated with the result that the liquid crystal exhibits ferroelectricity. Application of voltage to the ferroelectric liquid crystal causes the permanent dipole moment to be oriented in the direction of the electric field and simultaneously causes the whole molecules to be aligned. The ferroelectric liquid crystal in an SmC* phase has been widely used in displays. Ferroelectric liquid crystal imparts optical activity (chirality) to smectic liquid crystal itself, such as p-decyloxybenzylidene p′-amino 2-methylbutyl cinnamate (DOBAMBC) of which the molecular design and synthesis were made by R. B., Meyer et al. in 1975. Even in the case where an optically active compound itself does not have liquid crystalline properties (not a liquid crystal compound), addition of the optically active compound enables generation of an SmC* phase. In such a case, a liquid crystal matrix exhibiting a non-chiral smectic C (hereinafter referred to as SmC) phase is generally used.
Among smectic phases having layered structures, in the SmC* phase, the direction of the alignment of the liquid crystal molecules has a certain tilt with respect to the layer normal. In addition, the individual layers have slight difference in the tilt angle from a layer plane (direction angle), which produces a helical structure in the molecular alignment.
Display devices using ferroelectric liquid crystal have a response speed which is at least 10 times larger than that of display devices using nematic liquid crystal. The first ferroelectric liquid crystal applied to displays is surface-stabilized ferroelectric liquid crystal (SSFLC) made by Clark and Lagerwall. Ferroelectric liquid crystal has been intensively studied since this application. Besides ferroelectric liquid crystal, for instance, nematic liquid crystal is known, such as FLC (ferroelectric liquid crystal), a TM (twisted nematic) type, an STN (super twisted nematic) type, a DS (dynamic light scattering) type, a GH (guest-host) type, an IPS (in-plane switching) type, an OCB (optically compensated birefringence) type, an OCB (electrically controlled birefringence) type, a VA (vertical alignment) type, or a CSH (color super homeotropic) type.
In general, it is known that phenylpyrimidine readily exhibits a chiral smectic C phase, and phenylpyrimidine is therefore widely used in a ferroelectric liquid crystal composition. The composition containing phenylpyrimidine, however, has a problem in obtaining sufficiently high specific resistance, whereas a liquid crystal material having a high specific resistance is necessary for driving by a TFT when a ferroelectric liquid crystal composition is used, as in the case of using a nematic liquid crystal; thus, screen burn-in and flicker are likely to be caused, which has been problematic in terms of a reduction in the reliability of LCDs. In order to produce a highly reliable LCD driven by a TFT, a nematic liquid crystal material to which a fluorine substituent has been introduced and which has a high specific resistance is widely used because it is useful for, for example, liquid crystal television sets. In regard to a smectic liquid crystal having a fluorine substituent, there has been a report in which 2,3-difluoro-1,4-phenylene derivatives and terphenyls exhibit a smectic C phase (see Hon Patent Literatures 1 and 2). In particular, 2,3-difluoro-4-alkyloxy-4′-trans-4-alkylcyclohexyl ethyl)biphenyl does not exhibit a smectic C phase but has a high melting point (see Non Patent Literature 3).
A mixture of a variety of liquid crystal compounds containing highly linear liquid crystal molecules, such as 2,3-difluoro-1,4-phenylene derivatives and terphenyls, is used as a liquid crystal composition (e.g., see Patent Literature 1). Among such compounds, 2,3-difluoro-biphenyl derivatives have a problem in which birefringence Δn is high.
Almost all of known 2,3-difluoro-1,4-phenylene derivatives are, however, compounds having ring structures that are a trans-1,4-cyclohexylene group and a 1,4-phenylene group; unfortunately, such derivatives have a disadvantage in which they do not always have good compatibility with other liquid crystal compounds. It is therefore difficult to prepare a liquid crystal composition having a broad temperature range, particularly a liquid crystal composition which is less likely to suffer from crystal precipitation and phase separation even after being stored at low temperature for a long time; hence, there has been a need in which the melting point of a composition is decreased through an increase in solubility due to mixing of a number of various compounds with, for instance, a homologue (homologue having a difference only in the number of the carbon atoms of a side-chain alkyl group).
A liquid crystal compound has been studied as another technique that is different from the use of a homologue; the compound has enhanced properties specific to an n-type liquid crystal and enables expansion of the temperature range of a composition by introduction of a new skeleton structure different from a 2,3-difluoro-1,4-phenylene skeleton, such as a fluorine-substituted naphthalene structure or a tetrahydronaphthalene structure.
The addition of such a compound having a new skeleton structure, however, is highly restrictive in terms of, for example, compatibility with other liquid crystal compounds and effects on the physical properties and electro-optical properties of a liquid crystal composition, which has been problematic.